Sorption isotherm and length change behavior of autoclaved aerated concrete

We conducted an experimental study on the sorption isotherm and length change behavior of autoclaved aerated concrete (AAC) in both desorption and adsorption processes. Three types of AAC with different bulk densities were used to study the length change mechanism. The experimental results show that the expansion of AAC during the adsorption process is mostly larger than the shrinkage in the first desorption process even though the water content in the adsorption process is lower. Interestingly, a clear expansion in the desorption process in a range of relative humidity (RH) from about 90% to 65% and a significant difference in the length change strain between the desorption and adsorption processes (shrinkage hysteresis) at high RH are observed. Based on these experimental results, we propose a model to represent the mechanism of the hygral length change of AAC, taking into account the changes in surface free energy of the solid surface and capillary tension. The model reasonably explains the significantly different length change behaviors of AACs with different pore structures.